A structure in which a high refractive index material is surrounded by a material whose refractive index is lower than that of the high refractive index material acts as an optical waveguide that confines optical energy of light in the high refractive index region and its vicinity, and transmits the light in itself. A part whose refractive index is high is called a core, and a part whose refractive index is low is called a clad or cladding. The optical waveguide is applied to various optical components including those for optical communications.
Potassium tantalate niobate (KTa1−xNbxO3) is known as one example of the optical waveguide materials. KTa1−xNbxO3 (hereinafter referred to as KTN) is a material having the perovskite type crystal structure. FIG. 1 shows the unit cell of KTN crystal. When a simple cubic lattice having potassium ions on its lattice points is considered, an ion of tantalum or niobium is placed at its body center position, and oxygen ions are placed at its face center positions. KTN is a crystal material with a very large electro-optic effect that the refractive index varies upon application of an electric field (for example, see Patent Document 1). Moreover, since its refractive index will also vary when its composition is changed, an optical waveguide can be constructed with KTN by manufacturing a core and a clad each having a different composition.
The optical waveguide using KTN can modulate the phase of light being transmitted therein by means of the electro-optic effect by providing appropriate electrodes. Therefore, KTN can be used to manufacture optical components, such as an optical modulator etc., as is the case of LiNbO3 whose development for optical components has been preceded. Since KTN has a remarkably large electro-optic effect as compared with LiNbO3, KTN has the advantage of enabling optical components of higher performance in terms of low voltage operation etc. to be obtained.
In addition, K1−yLiyTa1−xNbxO3 (hereinafter referred to as KLTN) obtained by substituting Li for a portion of K of KTN is also a promising material that has the same perovskite type crystal structure as KTN and has a larger electro-optic effect than that of KTN.
However, with a change in a ratio of Ta and Nb in KTN and KLTN described above, the refractive index will vary, and at the same time the electro-optic coefficient and permittivity will change. For this reason, it was difficult to optimize characteristics of optical components by changing these parameters independently.
For example, when a refractive index difference between the core and the clad is set to 0.011 or more in order to enhance the performance of an optical waveguide element, the permittivities of the two members make a large difference. As a result, an electric field cannot be effectively applied to the optical waveguide, and hence an optical component making full use of a large electro-optic effect cannot be manufactured.
KTaO3 (hereinafter referred to as KT) has a small electro-optic effect compared with KTN and KLTN described above, but is known as an optical waveguide material of a high refractive index and is transparent for light of wavelengths down to 350 nm. KT is a crystal of an end member of KTN and KLTN, having the same perovskite type crystal structure as KTN and KLTN. That is, if a portion of Ta of KT is substituted for by Nb, the partially substituted KT becomes KTN. If a portion of Ta of KT is substituted for by Nb and a portion of K is substituted for by Li, the partially substituted KT becomes KLTN. Therefore, KT is used as a substrate material on which KTN or KLTN crystal layers are grown. If an optical waveguide is constructed using KT, its refractive index cannot be controlled by changing a ratio of Ta and Nb, because KT does not include Nb.
The object of this invention is to provide an optical waveguide material whose refractive index can be tailored without changing the ratio of Ta and Nb.
Patent Document 1: Japanese Patent application laid-open No. 2003-35831